194 Report on Trials of Plows. 



The strum upon a whiffle tree when used in plowing is analogous 

 to that of a l)eam supported at both its ends and sustaining a load 

 in the middle. The rule for estimating their strength is there- 

 fore the same in each case; and we know that the strength of 

 ])eams is proportional to their breadths multiplied into the square 

 of their depths and divided by their lengths. It is to be under- 

 stood that the depth here expressed is that dimension of the 

 whiffle tree which lies in the direction of the strain. It is used 

 to express what the farmer would call the breadth of the whiffle 

 tree. Suppose a whiffle tree three feet long between the trace 

 hooks; its depth three inches, and its breadth one and a half 

 inches, accordino; to the rule oiven above, we have ^ -^ ^ ^ = 4 • 5, 

 which, multiplied by the constant number 660 for oak, and 740 

 for ash, gives, in the one case, 2,970 pounds, and in the other 

 3,330 pounds, as tlie force that would break the whiffle tree. 

 The same rule is a})plicable to the evener. The strength, thus 

 found, is applica1)le onl^^ to the centre of the whiffle trees and 

 evener, for it is ol)vious that the strain at the extremities is only 

 half that at the centre. The ends may, therefore, be much less 

 in depth than the centre, with perfect safety. 



Wooden whiffle trees ought always to be fitted with clasp and 

 eye mounting of the l^est wrought iron, from two to two and a 

 half inches broad, about three-sixteenths of an inch thick in the 

 middle parts, and worked off to a thin edge at the sides. The 

 part forming the eye may range from one-half inch diameter in 

 the centre eye of the large tree to three-eighths inch in the end 

 clasps of the small trees; and they are applied to the wood in 

 a hot state, which, by cooling, embraces the wood very firmly. 



On the evener, the middle clasp has usually a ring or link 

 welded into it, by which it is attached to the hook of the plow 

 bridle. The two end clasps have their eyes on the opposite edge 

 of the whiffle trees, with sufficient opening in the eyes to receive 

 the (8) hooks of the whiffle trees and evener. 



The mode of attaching two horses to a plow is so well under- 

 stood by farmers that it is unnecessary to occupy any space in 

 describing it. 



Fig. 103 gives Mr. Stephens' mode of yoking three horses to 

 the plow, which works very well in practice. In this figure, a is 

 bridle of the plow; b the evener, five feet in length and of 

 strength proportioned to the draught of three horses; c d and e 

 are the three whiffle trees. The traces, though broken off in the 



